In a general gas turbine having a centrifugal compressor, the flow of air introduced into the turbine is accelerated by a rotor of the centrifugal compressor, and moves into a combustion chamber via a diffuser and a heat exchanger. In the combustion chamber, part of the air is converted into combustion gas by facilitating combustion of fuel. Then the combustion gas flows into a turbine room with the remainder of the air to rotate the turbine and the centrifugal compressor at high speed, and is thereafter discharged into the environment through an exhaust pipe. Part of the combustion gas flows via a space between the shaft of the centrifugal compressor and an engine casing into a space between the rotor disc of the centrifugal compressor and a partition wall in the engine casing, to flow behind the rotor disc as a secondary air flow. The secondary air flow joins the primary air flow from the rotor blade at the outer periphery of the rotor disc.
In a conventional large-sized gas turbine having a centrifugal compressor utilized in locomotives, ships and airplanes, the secondary air flow offers no problem to the efficiency of the centrifugal compressor since the secondary air flow is relatively small in comparison with the primary air flow and does not cause change of direction, or pressure loss in the primary air flow.
However, in a small-sized gas turbine having a centrifugal compressor utilized in an automobile, the secondary air flow causes change of direction and pressure loss in the primary air flow since the secondary air flow is relatively large in rate in comparison with the primary air flow. This is because the tip of the rotor blade is positioned on the same level with the outer periphery of the rotor disc. Namely, the secondary air flow joins the primary air flow at the portion right behind the tip of the rotor blade and increases the velocity and frictional loss of the primary air flow. Further, since the secondary air flow joins the primary air flow substantially at a right angle thereto, it changes the direction of the primary air flow and creates mixing, and the pressure loss of the primary air is increased.
As hereinabove described, the primary air flow does not have its direction changed by the secondary air flow when the secondary air flow is relatively small in comparison with the primary air flow, while the direction of the primary air flow is largely changed when the secondary air flow is relatively large. Consequently, the direction of the primary air flow deviates from a predetermined optimum inlet angle of the diffuser, resulting in an increase of pressure loss in the diffuser. The secondary air flow which is large in rate increases the velocity of the primary air flow to increase the frictional loss of the diffuser, and causes pressure loss by mixing with the primary air flow. Further, it lowers the efficiency of the gas turbine by moving the working area of the primary air flow.